1. Field of the Invention
One or more embodiments of the present invention relates to a simulated gas supply apparatus for supplying a simulated gas which is used for evaluating a catalyst or a sensor built in an exhaust pipe of a vehicle or the like.
2. Background Art
When a catalyst or a sensor built in an exhaust pipe connected with an engine of a vehicle is evaluated, concentrations of each component gas contained in an exhaust gas from the vehicle are preliminarily measured, and a simulated gas consisting of the same components as those of the exhaust gas is generated, and then the catalyst or the sensor is evaluated by using the simulated gas. For example, a mixed gas is generated by mixing CO, CO2, NO, NO2, HC and N2 gases supplied from each source and adjusting the flow rate of the gases, and then the mixed gas is heated and moisture is added to the mixed gas, whereby the simulated gas is generated. The simulated gas is supplied to a cell containing a test object such as the catalyst, and the concentration of the simulated gas is measured before and after the simulated gas passes through the catalyst, whereby purification performance of the catalyst is evaluated.
A NOx storage-reduction catalyst absorbs NOx when an air-fuel ratio of the exhaust gas is lean, and releases the absorbed NOx when the air-fuel ratio of the exhaust gas is rich. When the purification performance of the catalyst decreases, an internal combustion having the NOx storage-reduction catalyst changes operating conditions in such a way that the air-fuel ratio becomes rich. Thus, NOx contained in the exhaust gas can be purified (see Patent Document 1). In the internal combustion, NOx contained in the exhaust gas can be reduced by properly changing the lean and stoichiometric or rich conditions. The concentration of the exhaust gas is detected by a sensor, and the lean, stoichiometric and rich conditions are identified. NOx contained in the exhaust gas is detected by a NOx sensor, and the level of the purification of NOx is checked.
For an automobile exhaust gas regulation, some test modes are defined. For example, JC08 mode is well known in Japan and L4 mode is well known in U.S. When a test of the exhaust gas is carried out, a vehicle is placed on a chassis dynamometer and runs based on the defined test. An amount of each of air-pollution substances contained in the exhaust gas is measured based on a defined measurement method of the test. The air-pollution substances are measured by a gas analyzer, and a gas mileage of the vehicle is measured. Recently, the automobile exhaust gas regulations and the gas mileage standard become strict, whereby it is required that a catalyst evaluation test is carried out by using the simulated gas which changes in the same way as the change of the exhaust gas of the running vehicle based on the measurement mode.
In the conventional catalyst and sensor evaluation test, the simulated gas is changed by adding a rich or lean component to the simulated gas constantly supplied (see Patent Document 2). Thus, in order to change the concentrations of the simulated gas and the rich or lean component, the flow rates of each raw gas should be changed. However, when the flow rate of one of the raw gases is changed, those of the remaining raw gases are affected due to the change of the pressure in a flow pipe. In order to change the concentration and flow rate of the simulated gas, the flow rates of the raw gases should be changed. Thus, it takes several seconds to stabilize the flow rates of the raw gases. Each of the flow rates of the raw gases or each of the change ratios thereof are different from each other in accordance with required concentration and flow rate of the simulated gas, and time periods till when each of the flow rates of the raw gases become stabilized are different from each other, and therefore it is difficult to simulate the exhaust gas with high accuracy. Further, when the flow rates of the raw gases are changed, the concentration and flow rate of the simulated gas are fluctuated.    Patent Document 1: JP 2000-240429 A    Patent Document 2: JP 4194581 B